Saturday, June 16, 2007

Michael Purucker began researching the use of neural networks for predicting NFL games in 1996. He used 5 basic statistics based on each team’s performance over the previous 3 weeks: Total yards gained – total yards allowed, Rush yards gained – rush yards allowed, turnover margin (takeaways – giveaways through fumbles and interceptions), time of possession, and victories. Out of all the networks used on the problem, a back propagation network performed the best, achieving a 70.83% accuracy rate over weeks 14 and 15 of the 1994 season. Using the Las Vegas spread, which predicts the winner and the margin of victory, the results improved to 75%. Two weeks, however, is a very small test set. In the 2 weeks, the BP network with the spread was 9 of 14 and 12 of 14 respectively. Three games is a very big variance, and there’s nothing to guarantee it won’t go 6 of 14 in some weeks. The rush yards statistic overlaps with the total yards statistic, and both of the statistics are comparing a team’s offense with its own defense by subtracting yards allowed from yards gained. This does not reflect the matchups that actually take place on the field. The victory input does not take into account the margin of victory, and close games can come down to random events that would have let either team win.

Joshua Kahn continued Purucker’s study, testing statistics from the entire season in addition to the previous 3 weeks and eliminating the victories input. Kahn cites Purucker’s system as being 60.7% accurate over time. Kahn’s 3 week averages were 37.5 and 62.5% accurate over weeks 14 and 15 of the 2003 season respectively. Using season-long averages, Kahn achieved 75% accuracy in each of those two weeks, while the ESPN experts achieved 57% and 87% accuracy on average (~72% 2-week average). The study has the same problem of an extremely limited test set, but the results demonstrate an interesting point. Using season-long averages rather than 3-week averages yields a better predictor. This makes sense given the larger sample sizes involved. Teams that start off the season 0-3 almost never make the playoffs, so a 3 game winning streak that leads to a 3-10 record does not reflect the team’s quality. That the ESPN experts experienced a 30% variance in the 2 weeks, like the predictor using 3-week averages, could reflect how recent performance can skew human perception.

Roger Johnson uses only the Las Vegas spreads to formulate rankings for each team in the league and based on those rankings, predicts the winner of each game for weeks 3-17. For 2003-2006, the system has averaged about 64% accuracy, making it slightly less efficient than the simple “Las Vegas favorite wins” predictor. About 65% of the teams favored by Las Vegas in weeks 3-17 of the 1999-2006 seasons won.

Daniel Imamura’s “Computer Handicapper” takes more advantage of the data found in the box scores to produce efficiency ratings for various aspects of each team. Rather than just yards per game, many of the ratings are based on yards per play but also factor in turnovers and touchdowns. Using these metrics along with the Las Vegas spread, the handicapper predicts the winner and the margin of victory. Though primarily intended for use on betting with or against the spread, the system has been 55-68% accurate over the 2001-2006 seasons in simply predicting winners.

FootballOutsiders.com has gone beyond box scores and into play-by-play data and their own game charting project to devise an array of new statistics, the centerpiece being Defense-adjusted Value over Average (DVOA). The idea behind DVOA is that yards per game statistics are lossy data because the amount of yards gained in an individual play varies in true value based on the context of the down, yardage to go, field position, time left in the game, and the current score margin. In DVOA, each play is categorized as a success or failure and assigned some value based on the context. Given the baseline rates of success and the success value for each play’s context, a team’s overall performance is assigned a value over average, which is then adjusted for the opponent’s average performance. DVOA can be broken down into performance in any situation and by a certain subset of players, allowing for a very fine-grained evaluation of why a team is likely to win. The coarse team total DVOA, however, has shown to be a good predictor as well. In weeks 3-16 of the 2004 season, the team with the higher total DVOA won 67.3% of games. After week 17, the accuracy fell down to 65.625%, having predicted 7 out of 16 correct games. In the last week of the season, many playoff positions have already been decided, so some teams will rest their starters, which could skew the results. In 2005, the team with the greater DVOA won 66.67% of games, but in 2006, accuracy plummeted to 55.80%.

Injury reports categorize players as probable (P(Playing)=75%), questionable (50%), doubtful (25%), or out (0%). Using 1-P(Playing), an injury score is assigned to each player, and the score for each team is the sum of injury scores for all of its players. The team with the lower injury score won 52% of games in 2001-4. Teams with injury scores of 3 fewer points than opponents won 55% of games in that same time frame. When looking at changes in injury score from week to week, the team with the lower injury score "delta" won 55% of games. My original research included injury score inputs. In 2001-6, the team with the lower injury score won 51.947% of games (week 3-17 only). In the same time frame, the team with the lower delta won 52.036% of games. The obvious problem with the injury score is that it's not weighted for player values, but it's not entirely clear how to best do that.

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About the Author

My degree is in computer science, and the football research started as an independent study in artificial neural networks. As a lifelong NFL fan, I wanted to explore the relative importance of different factors in winning games. Since the research is still nascent, I wanted to put it out in the public domain and hopefully find others interested in teaming up. Once it becomes profitable, though... I just hope the mafia families running Vegas don't come to hurt me.